This application claims priority under 35 U.S.C. § 119 to Korean Patent Application No. 2006-04958, filed on Jan. 17, 2006 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates generally to sigma-delta fractional-N phase locked loops, and more particularly, to reducing lock time and frequency error in a sigma-delta fractional-N phase locked loop by gradual change of resistance within a loop filter.
2. Background of the Invention
A phase locked loop is used to maintain a stable frequency or to precisely change a frequency of a signal from a signal generator. Generally, the phase locked loop comprises a phase comparator, a charge pump, a loop filter, a voltage controlled oscillator, and a frequency divider.
The phase comparator generates control signals by comparing a reference signal and a feedback signal to determine phase difference. The charge pump generates a current (or charge) in response to such control signals and supplies the generated current to the loop filter. The loop filter filters such a current from the charge pump to generate a filtered voltage to the voltage controlled oscillator.
The voltage controlled oscillator generates an output signal having a frequency proportional to the filtered voltage from the loop filter. The frequency divider performs frequency division on the output signal of the voltage controlled oscillator by a predetermined factor to generate the feedback signal.
Recently, a phase locked loop that reaches the lock state quickly is especially desired from rapid development of wireless communications. Since lock time is inversely proportional to loop bandwidth of the phase locked loop, the loop bandwidth of the phase locked loop is increased for reducing lock time. However, the phase locked loop is more susceptible to noise at higher loop bandwidth.
For reducing the effect of such noise, when the phase locked loop is in the unlock state, the lock time is reduced by increasing the current of the charge pump circuit and reducing the resistance of the loop filter to increase the loop bandwidth. On the other hand, in the lock state, the lock time is increased by increasing the resistance of the loop filter and reducing the current of the charge pump circuit to reduce the loop bandwidth. However, reducing such loop bandwidth diminishes the susceptibility of the loop filter to noise.
FIG. 1 shows a charge pump 200 and a loop filter 300 of the above-described phase locked loop for reducing lock time in the unlock state and for improving noise immunity in the lock state. A controller 700 supplies a current control signal CCS and a phase margin control signal CRS to the charge pump 200 and the loop filter 300, respectively.
The loop filter 300 is connected between a ground node and a first node N1 that is an output terminal of the charge pump 200. The charge pump 200 generates a charge pump current with a controlled level thereby determining loop gain, in response to an up-control signal up and a down-control signal dn, from a phase comparator.
The loop filter 300 has a resistance therein that is adjusted in response to the phase margin control signal CRS. When the phase locked loop is in the unlock state, a third switch SW3 and a sixth switch SW6 are closed in response to the control signals CCS and CRS. When the up-control signal up is also activated (with the down-control signal dn being deactivated) to close switches SW1 and SW4, the closed switch SW3 increases the charging current of the charge pump 200 to be from the current sources CS1 and CS3.
Alternatively, when the down-control signal dn is also activated (with the up-control signal up being deactivated) to close switches SW2 and SW5, the closed switch SW3 increases the discharging current of the charge pump 200 to be from the current sources CS2 and CS4. In either case of increased charging current and increased discharging current, the loop bandwidth of the phase locked loop is increased in the unlock state.
When the sixth switch SW6 is closed, a resistor R1 is connected in parallel with another resistor R2 between a third node N3 and the ground node. Thus, the effective resistance of the loop filter 300 is reduced, and the bandwidth of the phase locked loop is increased in the unlock state. Such increased loop bandwidth of the phase locked loop reduces lock time of the phase locked loop.
When the phase locked loop reaches the lock state, the switches SW3 and SW6 are opened. Thus, the current level of the charge pump 200 is reduced, and the resistance of the loop filter 300 is increased. As a result, the loop bandwidth of the phase locked loop is reduced for in turn reducing noise susceptibility.
As compared to the phase locked loop, a fractional-N phase locked loop generates a signal with a frequency resolution less than a reference frequency. However, the fractional-N phase locked loop defectively includes an in-band spur as a noise component in the loop bandwidth. Therefore, a sigma-delta fractional-N phase locked loop is used with a shift of such a spur to a higher frequency.
FIG. 2A illustrates the change in bandwidth of the sigma-delta fractional-N phase locked loop by operation of the charge pump 200 and the loop filter 300 of FIG. 1. Referring to FIG. 2A, the loop bandwidth of the phase locked loop is increased for faster lock in the unlock state. Alternatively, the loop bandwidth is reduced for reduced susceptibility to noise in the lock state.
FIG. 2B illustrates in-band noise in the loop bandwidth of sigma-delta modulation noise (hereinafter referred to as “SDM noise”) when the loop bandwidth of the sigma-delta fractional-N phase locked loop is increased for faster lock in the unlock state. Referring to FIG. 2B, even after the noise spur generated by fractional-N division is sigma-delta modulated, the in-band noise is still widely present in the loop bandwidth (as illustrated by the shaded area in FIG. 2B).
FIG. 2C illustrates in-band noise of SDM noise when the loop bandwidth of the sigma-delta fractional-N phase locked loop is reduced in the lock state. Referring to FIGS. 2B and 2C, the in-band noise is reduced when the loop bandwidth is reduced (as illustrated by the shaded area in FIG. 2C).
FIG. 3A shows a simulation for frequency error of an output signal of the phase locked loop when the loop bandwidth of the sigma-delta fractional-N phase locked loop is increased to a wide bandwidth (20kHz) for faster lock in the unlock state and then reduced to a narrow bandwidth (2kHz) in the lock state. FIG. 3B shows an enlarged view of a dotted portion of the frequency error shown in FIG. 3A. Referring to FIG. 3B, the frequency error is larger for the wide bandwidth. The frequency error is reduced in the narrow bandwidth but lasts for a relatively long time after the lock state.
FIG. 4A illustrates a method for switching the loop bandwidth of the conventional sigma-delta fractional-N phase locked loop from a wide bandwidth to a narrow bandwidth. Referring to FIG. 4A, the loop bandwidth is reduced from 20 Khz to 2 Khz at once at time t1 by reducing the charge pump current from 32 mA to 2 mA and by increasing the resistance of the loop filter.
FIG. 4B shows the frequency error of the output signal of the phase locked loop when the loop bandwidth of the conventional sigma-delta fractional-N phase locked loop is switched from a wide bandwidth to a narrow bandwidth as illustrated in FIG. 4A. Referring to FIG. 4B, the frequency error lasts after the loop bandwidth is reduced, and the maximum frequency error after the reduction of the bandwidth occurs at time 851.7 usec with a frequency error of −4.947 Khz.
As the difference between the switched bandwidths becomes greater, the time needed for the frequency error to reach below a desired error range is extended. Thus, a sigma-delta fractional-N phase locked loop with reduction of such frequency error is desired.